Question: In history class, the girl to boy ratio is $2$ to $5$. If there are a total of $28$ students, how many boys are there?
A ratio of $2$ girls to $5$ boys means that a set of $7$ students will have $2$ girls and $5$ boys. A class of $28$ students has $4$ sets of $7$ students. Because we know that there are $5$ boys in each set of $7$ students, the class must have $4$ groups of $5$ boys each. There is a total of $20$ boys in history class.